Travelling wave tube



May 19, 1959 f w. D. MOBEE 8 TRAVELLING WAVE TUBE v Origirial Filed June9, 1954 61/027 A/GQ/V '50 2:90 am JaaSoqao'a'aa'Tmo @WK-MMVCAES P598561INVENTOR ATTORNEY U i ed St te Pw r'QT TRAVELLING WAVE TUBE Warren D.McB ee, Wantagh, N.Y., assignor to Sperry Rand Corporation, acorporation of Delaware 6 Claims. (Cl. 315-35) This invention relates totravelling wave tubes, and more particularly, is concerned with improvedmeans for coupling of radio frequency energy into and out of the helixof a travelling wave tube. This application is a continuation ofcopending application Serial No. 435,453, filed June 9, 1954, in thename of the present inventor, now abandoned.

As pointed out in the copending application Serial No. 426,362 filedApril 29, 1954, in the name of Seymour B. Cohn by the assignee of thepresent invention, to achieve a smooth transition between a helix andcoaxial line in a travelling wave tube for example, a tapered hornmember may be utilized which surrounds the end of the helix. The taperedhorn smoothly converts the helix mode of propagation to a TEM mode ofpropagation by gradually introducing a ground plane adjacent the helixwire. 'By proper spacing between the helix wire and the ground plane,the characteristic impedance of the transmission line in the regionwhere the TEM mode of propagation is eflected may be made equal to thecharacteristic impedance of the coaxial line coupled to the end of thehelix, so that a good match may be obtained over a substantial frequencyband.

The transition section between the tapered horn and the helix, at leastin the region where an appreciable amount of the energy is propagated inthe TEM mode, functions in the manner of a linearly tapered section ofcoaxial line. As is well known (see Microwave Transmission Design, by T.Moreno, McGraw-Hill, 1948, pages 5355), the VSWR of such a taperedsection varies at a given frequency as a function of the length of thetaper in a periodic fashion, with voltage minimums occurring when thelength of the taper is a half wavelength or integral multiple thereof. Asimilar periodic variation of VSWR exists with change in frequency for agiven length of taper, the voltage minimums occurrings where thefrequency is such that the length of thetapered section is a halfwavelength or integral multiple thereof. As the frequency increases,i.e., as the length of the taper in terms of wavelengths increases, thesuccessive maximums of VSWR decrease in magnitude. This indicates thatto operate within a certain maximum VSWR, at lower frequencies thelength of the taper must'be increased.

, 'lH ow ever, it is desirable that the horn in a travelling wave tubebe madeas short as possible. The reason is that the portion of the helixwithin the hoi'n does not interact with the electronostream, so that theeffective portion of the helix is only"that part betweenthe horns. Thusthe longer the horns are, .the longer must be the helix. and the.travelling wave tube to have the same 2,887,608 nt w te 959 ingstructure between the helix of a travelling wave tube and the input andoutput coaxial lines which for a given bandwidth of operation is morecompact and results in a less expensive tube.

These and other objects of the invention which will become apparent asthe description proceeds are achieved by the provision of a travellingwave tube having a helical conductor and means for directing a stream ofelectrons along the longitudinal axis of the helical conductor. A firsthorn member and a second horn member encircle the respective ends of thehelix and are coaxial therewith, the horn members having throat portionsof inner diameter slightly larger than the diameter of the helix andflaring portions directed toward each other. The flaring portion of eachof the horn members has two sections, the second section having agreater flaring taper than the first section and being larger indiameter where the two sections are joined. Thus, each of the hornmembers is provided with a step formed in the inner surface thereofbetween the two sections. Input and output coaxial lines couple the R.F.signal into and out of the helix at the ends thereof within the throatportions of the horn members.

For a better understanding of the invention, reference should be had tothe accompanying drawing, wherein:

Fig. 1 is a longitudinal view of a travelling wave tube showing thefeatures of the present invention;

Fig. 2 is a cross-sectional view taken substantially on the line 22 ofFig. 1; and

Fig. 3 is a series of graphs used in explaining the theory of operationof the invention.

Referring to Fig. 1, the travelling wave tube as there illustratedincludes at one end a cylindrical shell 10 forming part of the evacuatedenvelope. The input end of the tube includes a base 12 to which ismounted a cathode assembly 14. Connections to the cathode and heater(not shown) are made through suitable pin connections indicated at 16projecting from the base 12. An accelerating anode 18 is positionedwithin the shell 10 adjacent the cathode assembly 14, the anode 18having a central opening 20 therein provided with a grid assembly 22 toform in combination with the cathode 14 an electron gun assembly fordirecting a stream of electrons along the longituclinal axis of thetube.

Also mounted within the cylindrical shell 10 is a magnetic pole piece 24adjacent the anode 18. The pole piece 24 has an opening 26 aligned withthe opening 20 in the anode 18 along the longitudinal axis of the tube.The opening 26 is tapered over a portion of its extent in a manner andfor a purpose which will hereinafter be more fully described. Formingpart of the vacuum envelope and secured to the pole piece 24 is ametallic transition member 28 having a tapered opening 30 therethroughwhich is axially aligned with the openings 20 and 26. The anode 18 withits opening 20, the pole piece 24 with its opening 26, and thetransition member 28 with its opening 30 define an input horn memberhaving a throat portion and flaring portion. The flaring portion is intwo sections, the first section being formed by the flared part of theopening 26 in the pole piece 24 and the second section being formed bythe opening 30 in the transition member 28. A step 31 is providedbetween the two sections. This step is a significant feature of thepresent invention, as will become apparent.

The output end of the tube includes a collector electrode 32 and amagnetic pole piece 34, the collector electrode 32 being joined to thepole piece 34 by means of a cylindrical shell 36 forming part of theevacuated envelope of the tube. The pole piece 34 has an opening 38therethrough for passage of the electron stream directed towards thecollector from the cathode 14. Mounted adjacent the pole piece 34 isametallic horn member 40 forming part of the envelope of the tube, thehorn member 40 having an opening 42 therethrough extending along thelongitudinal axis of the travelling wave tube. As at the input end, themember 40 with its opening 42 defines an output horn member having athroat portion 43 and flaring portion 45 in two sections with a step '47between the two sections. The first section of the flaring portion 45extends from throat portion 43 to the step 47, the second sectionextending from step 47 towards the input horn member.

The input end and the output end of the tube are joined by an elongatedcylindrical metallic shell 44 extending between the transition member 28and the horn member 40. Extending along the longitudinal axis of thetube with a portion thereof being within shell 44 and coaxial therewith,is a helical conductor 46. The conductor 46 is supported by a pluralityof ceramic rods 48, the shell 44 having a diameter of sufficientmagnitude so that it has substantially no effect on the fields of energytravelling along conductor 46 over the desired frequency band ofoperation for the device. The rods 48 are preferably three in number.These rods extend between and are supported by the pole piece 24 at theinput end of the tube and the horn member 40 at the output end of thetube, the ends of the rod 48 being positioned in holes or bores 50 and52 in the pole piece 24 and the horn member 40 respectively. Thus, thehelix conductor 46 is coaxially positioned within the openings 26 and 30at the input end of the tube and the opening 42 at the output end of thetube. Ceramic spacer members 53 may be positioned along the helix togive support to the rods and helix.

An input coaxial line, indicated at 54, having an inner conductor 56 andan outer conductor 58, is provided for coupling ultra-high frequencyenergy into the traveling wave tube. To this end, the outer conductor 58passes through the shell and terminates in a T-junction with the anode13 and pole piece 24. The inner conductor 56 extends into the opening inthe anode 18 and is joined to the end of the helix 46.

Similarly, an output coaxial line section, indicated at 60, having aninner conductor 62 and an outer conductor 64, extends at right angles tothe horn member 40, the outer conductor 64 forming a T-junction with thehorn member and the inner conductor 62 extending into the longitudinalopening 42 where it joins the end of the helical conductor 46 forcoupling energy out of the helix. Both the input and output coaxial linesections preferably have a standard characteristic impedance of 50 ohms,the impedance between the helix 46 and throat portion of each of theinput and output horn members being substantially the same as that ofthe aforementioned coaxial line sections. The impedance of the sectionof helix 46 between the horn members is many times larger than that ofthe coaxial line sections at the lower frequencies within a desiredfrequency band of operation from 200-1000 megacycles, and becomesdecreasingly less as the frequency increases toward the upper end ofsaid frequency band.

Suitable magnetic means (not shown) may be provided in conventionalmanner to establish a magnetic focussing field between the pole pieces24 and 34. The aforementioned focussing field is provided formaintaining the electron beam produced by cathode 14 and anode 22 forpassage through helix 46 to collector 32 at a substantially constantdiameter as the beam progresses through the helix.

Within certain design'limitations to be described further below, thebest dimensions for the stepped horn over a particular frequency bandare generally developed empirically. A suitable horn and helix designwhich gives a VSWR of less than 2 over a 5:1 band and going down to 200megacycles at the low end is as follows:

First taper 640 Second taper 1040 Smallest dia. of first section 1.012"Axial length of first section Smallest dia. of second section 1.254Axial length of second section Helix outer dia 0.980" Helix turns perinch 5 By making the tapered portion of each horn of the presentinvention of two sections with a step discontinuity between rather thana straight horn of the same length without a step, it is possible toextend the bandwidth of operation at the lower frequency end of the bandwithout intolerably increasing the length of the horn and still operatewell below a VSWR (voltage standing wave ratio) design limit'of 2.0 atall frequencies within a frequency band of about 200-1000 megacycles.Why this is so may be better understood by referring to the graphs ofFig. 3 illustrating the VSWR vs. frequency curves of performance forvarious tapered horn helical transmission line matching sections.

Fig. 3a is illustrative of a typical frequency response curve of astraight tapered horn helix matching structure without a step, notillustrated, for operation over a desired frequency range from 200-1000megacycles. Such a horn is approximately that which would result from anextension in length of the first tapered section of the stepped hornillustrated in Fig. 1, whose design characteristics are given above, toapproximately 4 inches along a similar helix. This straight horn extendsalong its axis for about one-half wavelength of microwave energy alongthe helix at a frequency corresponding to the first VSWR minimum to theright of the ordinate in Fig. 3a.

The VSWR behavior of a horn and helix producing the results observed inFig. 3a is periodic with frequency up to about 575 megacycles, each VSWRminimum corresponding to a frequency at which the horn is approximatelyan integral number of half wavelengths long. Above 575 megacycles, thecurve in Fig. 3a smooths out. This is because at frequencies below about575 megacycles, reflection takes place from both ends of the hornproducing a typical interference pattern between the two reflectedwaves. Above the foregoing frequency, little or no reflection takesplace at the larger end of the horn since its dimensions were such thatits larger end was too far removed from the helix; i.e., the helixelectric field at the larger end of the horn is negligible for thehigher frequencies. If the straight tapered horn were made even longerthan that described above, the points of maximum VSWR would decrease inmagnitude. However, increasing the length of the horn is undesirable forreasons already described.

U A shorter, straight tapered horn than that described with reference toFig. 3a may also be used. The VSWR vs. frequency curve for the shorterhorn is illustrated in Fig. 3b, in which case the horn is aboutone-eighth as long as that producing the results shown in Fig. 3a with asimilar helix. The shorter horn gives good performance in a middle rangeof frequencies, but its performance is not as good as the long horn inthe upper half of the desired frequency bandof operation. The VSWR forthe short horn at lower frequencies below approximately 275 megacyclesis far above a desired minimum of 2. This is due to the fact that atlower frequencies, both ends of the taper of the short horn presentlarge discontinuities to the field and are spaced much less thanone-half wavelength apart so that one reflected wave does not cancel theother. An attempt to decrease the VSWR at lower frequencies byincreasing the horn length leads to a characteristic such as'that inFig. 3a. Horns of various lengths between the 0.5 inch horn producingthe results shown in Fig. 3b and the 4 inch horn producing the resultsin 2 at various points in the desired frequency band from 200-1000megacycles.

In the present invention, advantage is taken of the differences inradialvariation of the field at high and low frequencies by the twostage horn shown in Fig. 1 as above described to extend the range offrequencies for which the VSWR is less than 2' to a lower frequencylimit, and to keep the VSWR considerably less than 2 from this frequencylimit all the way up 'the band of frequencies including the highestfrequencies of operation. The VSWR vs. frequency performance curve ofthe' stepped horn and helix shown in Fig. l having' design dimensions asgiven above is shown in Fig.13c, thestepped horn showing goodperformance from 200 megacycles on up the frequency band. w

The first tapered section or stage of each stepped horn in Fig. 1adjacent. the throat thereof is designed so that the radius of itssmaller end corresponds to that of the throat portion. The first taperedsection extends along the axis of the tube for one half wavelength ofmicro wave energy carried by the helix within the horn at a frequency inthe upper;.-half of a desired frequency band. This frequency correspondsto approximately 680 megacycles per second for a horn design foroperation over a frequency band from approximately 200-1000 megacyclesper second producing the results shown in Fig. 30, for example. Theradius of the larger end of the aforementioned first tapered section ofeach horn is chosen so that the inner horn wall at the larger 'end ofthe first section is just beyond the effective radial. extent of thehelix fields at a predetermined frequency slightly lower. than that atwhich the first tapered horn section would become three quarters of awavelength'long. The foregoing predetermined frequency corresponds toapproximately 1000 megacycles per second for a stepped horn providing aperformance curve over the frequency band shown in Fig. 3c.

The smaller end of-the second tapered section of each horn is slightlylarger than the adjacent end of the first tapered section so that a stepdiscontinuity exists between the sections. The extents Of IbOth thefirst and second tapered sections of each horn along the axis of thetube is chosen so that their total is approximately one-half wavelengthof the microwaves carried by the helix within the horn at a frequencywithin the lower half of the design frequency range for the device. Inthe example shown in Fig. 3c, the foregoing frequency corresponds toabout 290 megacycles per second, for example. The largest end of thesecond tapered section has a radius so that the inner wall of the largeend of the horn is just beyond the effective radial extent of the helixfields at a predetermined frequency slightly lower than that at whichthe total lengths of both horn sections would become three quarters of ahelix wavelength long.

The taper of the second section of each horn is greater than the taperof the first tapered section thereof, the size of the step between thetapered sections being chosen so that it is just sufficient for insuringthat the second section of horn including the smaller end of said secondsection has substantially no effect upon the operation of the device forfrequencies higher than the frequency at which the first tapered sectionis one-half wavelength long. If the step between the tapered sections istoo large, the impedance match provided by the horn is adverselyaffected at the low frequency end of the frequency hand. If the step ismade too small, an interference peak shows up at the middle frequencies.The size of the step must be designed within the foregoing limitations.

When it is stated above that a portion of the horn has a certain radiusso that the inner horn wall thereat is just larger than the effectiveradial extent of the helix field at a certain frequency, it is meantthat E of the helix electric field E at such an extent is negligible;i.e., has a band, said horn means extending axially of said helix for 6strength whichis at least 10 db 'less than the strength" of the E 'ofthe electric field at the helix.

For purposes of calculating the length of the horn sections for thedevice of Fig. 1 in accordance with the above described designprocedure, the phase velocity around the ;where P is the distancebetween turns of the helix, C is the velocity of light, ris the radiusof the helix and n" is 3.1416.- The length of the waves A, along thehelix is f a v. where 7 represents the operating frequency. It should bekept in mind, however, that the velocity of propagation around the turnsof the helix becomes decreasing less than the velocity of light as thefrequency decreases since the horn has more effect on the velocity ofthe helix waves at lower frequencies, especially in the lower half ofthe desired operating frequency band.

. From the above description, it will be seen that the various objectsof the invention have been achieved by the provision of an improvedtransition between a helix and a coaxial line for use in a travellingWave tube. By providing a horn of two tapered sections with a stepbetween, the length of the horn transition section has been materiallyreduced over that required to achieve similar matching qualities at thesame frequencies with a smooth 1y tapered horn. In fact the length of astraight horn achieving an equivalent match to that of the presentinvention is many times that of'the stepped horn described herein.

While theinvention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than of limitation and that changes within thepurview of the appended claims may be made without departing from thetrue scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In combination, a conductive helix for guiding travellingelectromagnetic wave energy over a predetermined frequency band,transmission line means coupled to one end of said helix, saidtransmission line means having an appreciably dilferent value ofimpedance from that of said helix "over said frequency band, and taperedhorn means extending coaxially along and surrounding a portion of saidhelix beginning near said one helix end for providing an efiicienttransfer of energy between said transmission line means and helix oversaid frequency substantially one half wavelength of helix wave energy ata given frequency within said band, the end of said tapered horn meansof largest radius being furthest from said one end of said helix withsaid radius being just larger than the effective radial extent oftravelling wave energy along said helix at a higher frequency than saidgiven frequency within said band at which said horn means extendsaxially of said helix for approximately three quarters of a wavelengthof helix wave energy.

2. In combination, a conductive helix for guiding travellingelectromagnetic wave energy over a predetermined frequency band,transmission line means of different impedance from said helix forelectromagnetic energy within said frequency band being coupled to oneend of said helix, and tapered horn means extending coaxially along andsurrounding a portion of said helix beginning near said one helix endfor providing an efficient transfer of energy between said transmissionline means and helix, said horn means having first and second taperedsections whose sum total of axial extent along'the axis of said helix issubstantially one half wavelength of helix wave energy at a firstfrequency at the lower end of said band, the radius of the end of saidhorn means farthest from said one helix end being just larger than theeffective radial extent of helix wave energy at a second frequencywithin said band at which said horn means extends axially of said helixfor three quarters'of a Wavelength of helix wave energy, the firstsection of said horn means extending axially of said helix forsubstantially one half wavelength of helix wave energy at a thirdfrequency within said frequency band higher than said first and secondfrequencies, the radii of the adjacent ends of said first and secondhorn sections being larger than the effective radial'extent of a helixwave energy at a fourth frequency within said band at which said firstsection of horn means extends axially of said helix three quarters of awavelength of helix wave energy. 7

3. The combination as set forth in claim 2, wherein the smaller end ofsaid second sectionof said horn means has a larger radius than theradius of the larger end of said first section of horn means and iseffectively beyond the effective radial extent of helix 'wave energy atsaid third frequency for which said first section extends axially ofsaid helix for substantiallyone half wavelength of helix wave energy,the radius of the larger end of said first section of horn being lessthan the effective radial extent of helix wave energy at said thirdfrequency.

4, A travelling wave tube for operation over a wide microwave frequencyband between predetermined frequency limits, comprising conductive helixmeans extending along a predetermined axis, first and second meanslocated at opposite ends of said helix means for coupling microwaveenergy over said frequency band to and from said helix means,respectively, means for producing and directing a stream of electronsalong said axis for interaction with microwave energy propagated by saidhelix means between said first and second coupling means, and first andsecond conductive tapered horn members adjacent respective ones of saidcoupling means in coaxial relationship with said helix means andencircling opposite end regions of said helix means between saidcoupling means; each of said horn members having a flared portiondivided into first and second sections by a step between the larger endof said first section and the smaller end of said second section, theradius of the smaller end of said first section being slightly largerthan the radius of said helix means with the impedance between saidhelix means and horn member at the smaller end of said first sectionbeing substantially the same as that of the coupling means adjacentthereto for microwave energy within said frequency band, the sum of theaxial lengths of both sections of each horn being one half wavelength ata first frequency within the lower half of said frequency range, theradius of the larger end of said second section of each horn being justlarger than the effective radial extent 'of a helix field at a secondfrequency slightly lower than that which the total-axial extent of saidfirst and second sections of each horn is three quarters of awavelength, the axial length of said first section of each hornbeinga'pproximately one half of a helix means wavelength at a thirdfrequency in the upper half of said frequency band, the radius of thelarger end of said first section of each horn being just larger than theeffective radial extent of the helix field at a fourth frequencyslightly lower than that at which said first section of horn isapproximately three-quarters of a helix wavelength long,

said second section of horn being outside the effective radial extent ofhelix fields at said third frequency at which said first section of eachhorn is one-half wavelength long.

5. A travelling wave tube device, comprising a conductive helix, meansfor coupling R.F. energy over a predetermined frequency band into andout of the helix at respective ends thereof, means for producing anddirecting a stream of electrons along said helix for interaction withhelix R.F. energy, first and second horn members encircling oppositeends of said helix and coaxial therewith for enhancing the transfer ofenergy between said helix and said R.F. coupling means, each of saidhorn members having first and second tapered sections of increasinglylarger diameter extending along the axis of said helix with the radii ofadjacent ends of said tapered sections being different, the axial extentof said first tapered sections of each horn being one half wave lengthat a predetermined operating frequency within said frequency band, thewalls of the second section of each horn being beyond the effectiveradial extents of helix fields at said predetermined frequency and abovewhile being within the effective radial extent of helix fields at lowerfrequencies.

6. A travelling wave tube as set forth in claim 5, wherein the taperedsection of each horn member most remote from the smaller end of the hornhas a larger taper than the other tapered 'section thereof, the sumtotal of the axial extents of both tapered sections of each horn beingone half wavelength for R.F. helix energy at a lower frequency withinsaid frequency band than said predetermined frequency,'the radius ofeach horn member at its largest end being just larger than the effectiveradial extent of RF. helix energy at a frequency within said frequencyband at which each horn member is approximately three quarters of awavelength long for RF. helix energy.

References Cited in the file of this patent UNITEDSTATES PATENTS2,578,434 Lindenblad Dec. 11, 1951 2,643,353 Dewey June 23, 19532,645,737 Field July 14, 1953 2,727,179 Lally et al Dec. 13, 19552,765,423 Crapuchettes Oct. 2, 1956

